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[en] Mass imbalance in strongly interacting mixtures of ultracold fermions is predicted to lead to new pairing phenomena and quantum phases. We investigate a mass-imbalanced ⁶Li-⁴⁰K Fermi-Fermi mixture in the regime of strong interactions on the repulsive side of an interspecies Feshbach resonance. We find that, for a sufficiently strong repulsive s-wave interaction, the ⁴⁰K atoms and the ⁶Li⁴⁰K dimers interact attractively, which is in strong contrast to the mass-balanced case. This surprising behavior is related to the existence of a ↑↑↓ trimer state in ↑↓ Fermi-Fermi mixtures with a mass ratio m_↑/m_↓ > 8.2. For lower mass ratios (i.e. m_K/m_Li = 6.64) this trimer state turns into a p-wave atom-dimer scattering resonance. Here, we present our experimental results on interactions in a resonantly interacting atom-dimer mixture. Employing radio-frequency spectroscopy over a range of temperatures and interaction strengths, we confirm the presence of a strong attraction on the repulsive side of a Feshbach resonance, in good agreement with theory.

[en] We perform extensive magnetic Feshbach spectroscopy of an ultracold mixture of fermionic ⁴⁰K and bosonic ⁸⁷Rb atoms. The magnetic-field locations of 13 interspecies resonances are used to construct a quantum collision model able to predict accurate collisional parameters for all K-Rb isotopic pairs. In particular, we determine the interspecies s-wave singlet and triplet scattering lengths for the ⁴⁰K-⁸⁷Rb mixture as (-108±3)a₀ and (-205±5)a₀, respectively. We also predict accurate scattering lengths and the position of Feshbach resonances for the other K-Rb isotopic pairs. We discuss the consequences of our results for current and future experiments with ultracold K-Rb mixtures

[en] In this review, we discuss the properties of a few impurity atoms immersed in a gas of ultracold fermions—the so-called Fermi polaron problem. On one hand, this many-body system is appealing because it can be described almost exactly with simple diagrammatic and/or variational theoretical approaches. On the other, it provides a quantitatively reliable insight into the phase diagram of strongly interacting population-imbalanced quantum mixtures. In particular, we show that the polaron problem can be applied to the study of itinerant ferromagnetism, a long-standing problem in quantum mechanics. (report on progress)

[en] We discover several magnetic Feshbach resonances in collisions of ultracold ³⁹K atoms, by studying atom losses and molecule formation. Accurate determination of the magnetic-field resonance locations allows us to optimize a quantum collision model for potassium isotopes. We employ the model to predict the magnetic-field dependence of scattering lengths and of near-threshold molecular levels. Our findings will be useful to plan future experiments on ultracold ³⁹K atoms and molecules